1. Field of the Invention
The present invention relates generally to a circuit for calibrating and correcting the pulse amplitude output of a pulse generator, and more particularly pertains to a calibration circuit for adjusting and correcting the output pulse high voltage level and low voltage level.
2. Discussion of the Prior Art
The calibration circuit of the present invention is associated with that part of a prior art pulse generator circuit which determines the output pulse high and low voltage levels, and the rise and fall times of the output pulse. In operation of such a pulse generator circuit, a logical "1" state of an input pulse causes a current switch to direct a leading edge current supplied by a leading edge current generator to charge a capacitor until a high voltage level is reached. When the high voltage level is reached, the leading edge current flows into a positive clamping circuit thereby clamping the high voltage level. When the state of the input pulse is a logical "0", the current switch directs the leading edge current to flow to ground such that it no longer affects the capacitor voltage. A trailing edge current supplied by a trailing edge current generator is then directed to flow to the capacitor causing it to charge in a negative direction until a low voltage level is reached. At that point, the trailing edge current is directed into a negative or low clamping circuit and the capacitor voltage is clamped to the low voltage level.
The high voltage and low voltage clamping circuits determine the accuracy of the pulse upper output voltage and the pulse lower output voltage The accuracy of the clamping circuits depends upon the characteristics of a pair of transistors therein being precisely matched so that when equal emitter currents flow through each, there will also be equal base voltages and equal collector voltages across each. The aformentioned matching requirements cause many practical problems. If the transistors are to switch at high speeds as required by a pulse generator of short pulses at high pulse repetition frequencies, then they generally are not available in matched pairs, but must be purchased in volume, and then characterized and matched by the equipment builder. This task is unattractive because of the substantial effort involved, and also because of the fact that many transistors will be left over because they cannot be matched over the large current range required. The present invention makes the transistor matching requirements much less stringent, and consequently increases the yield of transistor pairs and decreases the selection effort.
An output amplifier amplifies the clamped pulse output voltages and to amplify high frequency pulses, the amplifier must be capable of amplifying high frequencies and also all lower frequencies including DC. The DC requirement results in a contribution to the output voltage error which is the DC offset of the output amplifier. This offset normally varies with time and changes in the amplification factor itself. The present invention compensates for output amplifier errors as well as for clamping circuit errors as the two errors are cumulative.